Sprue (manufacturing)

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Bronze casting showing sprue and risers

A sprue is the passage through which liquid material is introduced into a mold. During casting or molding, the material in the sprue will solidify and need to be removed from the finished part. This excess material is also called a sprue.


In casting, a sprue is the passage through which a molten material is introduced into a mold, and the term also refers to the excess material which solidifies in the sprue passage. In sand casting, the sprue is formed by a dowel which is removed from the sand to make the hole into which the metal is poured.


Sprues can serve as filters, heat sinks, and as feeders. Bronze in particular has a high shrinkage rate as it is cooling; a sprue can continue to provide molten metal to the casting, provided it is large enough to retain its heat and stay liquid, as metal in the main casting cools and shrinks. The design of the sprue and runner system can be also utilized to trap unwanted dross and sand from continuing into the main cavity; this may include adding porous material to the runners, or designing the sprue to eject the dross to the side of the sprue using cyclonic separation.

Sprue design[edit]

The design of gating and runner is also essential for casting. The design can be done by using vertical and bottom gating.

For bottom gating

t_f = \frac {2 A_m(\sqrt {h_t}-\sqrt {h_t-h_m})} {A_g \sqrt{2g}}


t_f = Time for filling
A_m = Area of mold
A_g = Area of gate
g = Acceleration due to gravity
h_t = Total height
h_m = Height of mold cavity

This equation may change if the height of gating is equal to height of casting material.

Then the equation will be:

t_f = \frac {2 A_m\sqrt {h_t-(h_t-h_m)}} {A_g \sqrt {2g}}

or, simplified,

t_f = \frac {2 A_m\sqrt {h_m}} {A_g \sqrt {2g}}


t_f = Time for filling
A_m = Area of mold
A_g = Area of gate
g = Acceleration due to gravity
h_t = Total height (Height of gating + height of mold cavity)
h_m = Height of mold cavity

Injection molding[edit]

In injection molding, the term sprue refers to the passages through which a liquid material (such as polystyrene or polyvinyl chloride) flows into a die, where it solidifies to form parts. It also refers to the material which solidifies in these passages, forming a framework attaching the parts together in a roughly planar arrangement.

Gates and runners[edit]

Some moldmakers make a distinction between three separate entities: the gate, the runner, and the sprue. Certainly in the plastics injection moulding industry, the gate is the location at which the molten plastic enters the mold cavity and is often seen as a small nub or projection (the "gate mark") on the molded piece. The sprues are large-diameter channels through which plastic flows, usually around the edges of the part or along straight lines. Finally, in this naming scheme, the runner represents only the smaller channels that divert from the sprue to the individual part. An analogy to the sprue/runner system might represent the sprues with city water mains, and runners with the smaller pipes leading to individual houses.

Many scale model kits are made from injection-molded plastic. Hobbyists, such as builders of scale models, typically remove the parts of a model kit from the runner using a sharp craft knife or razor saw.

Model makers sometimes use the sprue or runner as a raw material to fabricate additional parts, such as railings on model ships, antenna wires on airplanes, or greebles on fictional spacecraft.

Sprues in model kits often include engravings to identify the parts by number.

See also[edit]